drm/ast: Move Gen2+ and Gen1 POST code to separate source files

Move POST code for Gen2+ and Gen1 to separate source files and
hide it in ast_2100_post() ans ast_2000_post(). With P2A
configuration, the POST logic for these chip generations has
been mingled in ast_init_dram_reg(). Hence, handle all generations
in a single change. The split simplifies both cases. Also move
the DRAM init tables for each Gen into the respective source
file. No changes to the overall logic.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-6-tzimmermann@suse.de